Increased global demand for energy, along with widening regulation of carbon dioxide emission, have expanded interest in renewable energy sources and improved efficiencies in fuel use. However, an important restriction regarding the adoption of new fuels and energy technologies is the cost of power produced by these means. While subsidies and other forms of artificial support may assist in the introduction of these renewable energy sources, successful displacement of traditional fuels must necessarily be driven by total energy costs.
Pressure retarded osmosis (PRO), or “salinity power” as it is often referred to, is a membrane-based osmotic pressure energy conversion process. PRO utilizes osmotic flow across a semi-permeable membrane to generate electricity. PRO processes are discussed for example in U.S. Pat. No. 3,906,250 to Loeb, U.S. Pat. No. 3,587,227 to Weingarten et al., and U.S. Pat. No. 3,978,344 to Jellinek, the subject matter of each of which is herein incorporated by reference in its entirety.
At first, locations considered suitable for using PRO technology focused on river deltas at saline water bodies such as the ocean, Dead Sea or the Great Salt Lake. At these locations, an osmotic pressure gradient exists where freshwater from a river freely mixes with seawater. The PRO process utilizes this chemical energy and converts it into electricity. In the prior art PRO processes, saline water is pressurized and placed opposite of the freshwater water across a semi-permeable membrane. The osmotic pressure difference between the seawater and the freshwater (which is greater than the hydraulic pressure induced on the seawater) causes osmotic flux to occur across the membranes. As flux occurs into the pressurized seawater, the pressure is relieved by expansion through a hydroturbine (or other means) which generates electricity.
PRO processes at river deltas, also know as “open loop” PRO, have several operational and design limitations. First is the need for extensive pretreatment of feed and draw streams, similar to that required in desalination processes, to prevent fouling of process membranes and components.
Another difficulty arises from the low differential osmotic pressures found between many natural feed waters. That is, the available osmotic pressure difference is not extraordinarily high unless the saline water body is hypersaline, such as the Dead Sea or the Great Salt Lake. Unfortunately, the volumetric flow of water into these water bodies is somewhat small and hence will yield limited power for even a well designed PRO process. Sea water, for example, has an osmotic pressure of approximately 2.53 MPa (25 atm), which does not allow for the high hydraulic pressures that are desirable for efficient power production. In cases where higher concentration streams are considered, high hydraulic pressures may be used, but the process efficiency will suffer significantly from internal concentration polarization (ICP) which occurs in the support structure of the membrane used for the process. This phenomenon is particularly exacerbated by the increased support layer thickness required to resist the increased hydraulic pressures enabled by more concentrated streams.
A final consideration is the need to place power facilities at the interface between natural streams, often areas considerable environmental importance, such as estuaries, wetland and bays.
However, the primary obstacle to a viable PRO process is poor membrane performance. Previous investigations into PRO have found that membrane flux performance was too poor to make power generation a viable option. Low flux rates require the use of more membrane area to achieve enough volumetric flow to generate power and are due to a phenomenon called concentration polarization.
Flux occurs from the dilute “feed” solution (freshwater) into the concentrated “draw” solution (sewater). As this happens, solutes build up along the surface of the membrane along the feed side. On the permeate side of the membrane, solvent dilutes the dissolved solutes along the membrane surface. Since the solute concentrations at the membrane surface dictate the true osmotic pressure difference across the membrane, these concentration polarization phenomena must be minimized to ensure high fluxes. The severity of the concentration polarization phenomena can be mitigated by crossflow, wherein turbulent flow near the membrane surface can reduce the thickness of these boundary layers.
Unfortunately, membranes currently in use are asymmetric in structure. In these membranes, a thin separating layer (the layer which rejects the salt, also called the “active layer”) is supported by a porous support layer which provides mechanical strength to the membrane. These membranes have been designed for pressure driven membrane processes, such as reverse osmosis (RO). In reverse osmosis, these support layers do not inhibit flow since water is literally being forced through the membrane by hydraulic pressure. On the other hand, in osmotic flow, the osmotic pressure driving force is established only over the thin active layer. The porous support layer plays a significant, and often hindering, role in osmotic flux performance.
As illustrated in FIG. 1, a significant concentration polarization layer can form within the porous support layer on the feed side. Called “internal concentration polarization” (ICP), this layer impacts the osmotic pressure to a much greater extent than concentration polarization layers external to the membrane (ECP). Minimization or elimination of ICP is critical for viable performance of pressure retarded osmosis. The membrane must still be able to reject salt to a high degree, however and be highly permeable to water.
For PRO applications, the draw solution must have a high osmotic pressure in order to generate reasonable amounts of power. In river delta PRO, however, the osmotic pressure gradients are rather small. Smaller osmotic pressure gradients require more membrane area to generate large volumetric flows. This problem, coupled with ICP and fouling phenomena make the available osmotic pressure even smaller. Other issues associated with the draw solute include compatibility with the system components and the membrane. Seawater may be corrosive to metal parts and both freshwater and seawater may contain biological components that cause biofouling to system components, including the membrane.
River delta PRO also runs in an open-loop configuration. This means that the feed and the draw solutions are returned to the ocean after the PRO process is complete. When the seawater and river water are brought into the PRO system, they must be filtered and disinfected to prevent fouling and biofilm formulation, respectively. In addition to adding to the overall cost of the project, any chemicals that are added to these waters must either be flushed out to sea or be removed through physical or chemical means. Disposal of disinfection chemicals and disinfection byproducts can have unforeseen environmental impacts. Diversion of river water may also have an environmental impact on delicate river delta ecology.
Thus, in order to create a viable pressure-retarded osmosis process, the use of closed cycle PRO systems, which are intended to use low temperature heat to recycle an osmotic agent, have been proposed. This approach does not capitalize on natural salinity gradients but instead explores the use of osmotic pressure as a medium for the production of work, enabling the conversion of environmentally benign low temperature heat sources to electrical power. In several processes, the draw solution is a solution of an ioinic salt, such as sodium, chloride, as described for example in U.S. Pat. No. 3,906,250 to Loeb. Heat applied to the OHE would re-concentrate the draw solution by vaporizing a portion of the water into steam, which would then be condensed to form the de-ionized working fluid. Other processes involve the removal of a volatile organic solute, or the chemical precipitation of solutes followed by their re-dissolution.
A primary difficulty faced by these OHEs is poor thermal efficiency due to high heat input requirements for water and organic solute vaporization. In the case of chemically precipitable solutes, chemical feed stock consumption can pose difficulties to economic operation. An additional challenge is the difficulty of obtaining solute separation complete enough to avoid concentration polarization (CP) effects in the feed water. This is not a problem when water is vaporized and re-condensed as distilled working fluid, hut could pose a significant problem when using removable draw solutes which are difficult to remove completely.
This points to an additional, reoccurring challenge in osmotically driven membrane processes—the difficulty of identifying a solute which may both create high osmotic pressures and be highly removable for reuse. Near complete removability is very important, because internal concentration polarization effects in the working fluid (feed solution) can drastically reduce membrane water flux. Thus, the ideal osmotic heat engine would use a draw solute that has the following features; (1) highly soluble; (2) completely removable; (3) has a high diffussvity for effective mass transfer in the membrane system, and (4) requires less heat for solute removal that that required for the vaporization of water or highly soluble organic solutes.
The invention described herein attempts to overcome some of the noted problems of the prior art by proposing an alternative means of power production, that use osmotic pressure to generate electrical power from sources of low-grade heat. While several prior investigations of the use of osmotic phenomena to produce power have been conducted, such as those used to convert “salinity power” from the mixing of natural saline and fresh water streams, relatively few studies have focused on the use of osmotic phenomena to produce power through the conversion of heat.